Laser refrigeration of hydrothermal nanocrystals in physiological media

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-12, Vol.112 (49), p.15024-15029
• Main Authors: Roder, Paden B., Smith, Bennett E., Zhou, Xuezhe, Crane, Matthew J., Pauzauskie, Peter J.
• Format: Article
• Language: English
• Subjects: Biophysics; Fluorides - chemistry; Lasers; Lithium Compounds - chemistry; Nanocrystals; Nanoparticles - chemistry; Physical Sciences; Radiation; Refrigeration; Scattering; Telecommunications; Yttrium - chemistry
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1510418112

Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose–Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10 °C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from 1 μm. A tunable, nearinfrared continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm². Heat is transported out of the crystal lattice (across the solid–liquid interface) by anti-Stokes (blue-shifted) photons following upconversion of Yb³⁺ electronic excited states mediated by the absorption of optical phonons. Temperatures are quantified through analysis of the cold Brownian dynamics of individual nanocrystals in an inhomogeneous temperature field via forward light scattering in the back focal plane. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 °C below ambient conditions in D₂O, suggesting a range of potential future applications including single-molecule biophysics and integrated photonic, electronic, and microfluidic devices.